US 2008/0059037 (JP 2008-062672) proposes a vehicle speed control system, which controls vehicle travel speed by using, as a distance condition relative to an object present in a forward direction of a subject vehicle, an evaluation index indicating an approach/separation condition and determined based on a time change rate of a seeming area of a forward object projected on retina of a driver of the subject vehicle.
In this system, a target travel speed Vs0—t of the subject vehicle for traveling a curving road is set based on a present value Gy_p of a lateral acceleration G of the subject vehicle when traveling the curving road and a radius of curvature R of the curving road. When a difference between the target speed Vs0—t and a present travel speed Vs0 of the subject vehicle is greater than a predetermined difference, a target acceleration/deceleration speed dVs0dt for accelerating or decelerating the subject vehicle to the target speed Vs0—t is determined based on the present evaluation index KdB_p indicating the approach/separation condition of the subject vehicle, a distance D between the subject vehicle and a fixture attached to the road in the forward direction of the subject vehicle, the present speed Vs0 of the subject vehicle and the target speed of the subject vehicle Vs0—t. Thus, the acceleration and deceleration of the subject vehicle is automatically controlled to the target acceleration/deceleration speed dVs0dt. 
According to the above-proposed system, the radius of curvature R is determined as a value greater at a position before an entrance of the curving road as the distance D between the subject vehicle and the fixture of the road increases than at a position of the entrance of the curving road. As a result, the target speed Vs0—t for traveling the curving road ahead the subject vehicle is determined to be a greater value. The radius of curvature R at the curving road is calculated to be less at the entrance position of the curving road. As a result, the target speed Vs0—t for traveling the curving road ahead the subject vehicle is determined to be less.
For this reason, the difference in the speed between the actual speed Vs0 and the target speed Vs0—t of the subject vehicle is not reduced sufficiently. It is thus not possible to perform the speed control, which matches actual sense of a driver of the subject vehicle entering the curving road.